fix(exclusions): fail safe when Mdblist is unavailable

[rfsbraz]

Fixes #288

Stacked on #283 - merge that first; this PR extends its _handle_library_failure machinery.

• Mdblist fetch failures (network error, rate limit, HTTP 5xx) previously logged one line and returned a partial or empty list, which the caller treated as the complete exclusion set - items on the user's protection list became deletable during an mdblist outage
• _fetch_list_items now raises a typed MdblistError instead of returning partial results; a misparsed list URL (config mistake) also raises instead of silently disabling the exclusion
• The per-library guard catches it and skips the library for the run (fail safe), matching the Trakt behavior from fix(exclusions): fail safe when JustWatch or Trakt are unavailable #283
• Existing tests that pinned the fail-open behavior flipped to assert the fail-safe contract; added a deleterr-level test that an mdblist failure skips the library with zero deletions

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

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